Department of Geography, University of Zurich, Zürich, Switzerland.
Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland.
Glob Chang Biol. 2022 May;28(9):2956-2978. doi: 10.1111/gcb.16136. Epub 2022 Mar 7.
Extreme events such as the summer drought of 2018 in Central Europe are projected to occur more frequently in the future and may cause major damages including increased tree mortality and negative impacts on forest ecosystem services. Here, we quantify the response of >1 million forest pixels of 10 × 10 m across Switzerland to the 2018 drought in terms of resistance, recovery, and resilience. We used the Normalized Difference Water Index (NDWI) derived from Sentinel-2 satellite data as a proxy for canopy water content and analyzed its relative change. We calculated NDWI change between the 2017 pre-drought and 2018 drought years (indicating resistance), 2018 and the 2019 post-drought (indicating recovery), and between 2017-2019 (indicating resilience). Analyzing the data from this large natural experiment, we found that for 4.3% of the Swiss forest the NDWI declined between 2017 and 2018, indicating areas with low resistance of the forest canopy to drought effects. While roughly 50% of this area recovered, in 2.7% of the forested area NDWI continued to decline from 2018 to 2019, suggesting prolonged negative effects or delayed damage. We found differential forest responses to drought associated with site topographic characteristics and forest stand characteristics, and to a lesser extent with climatic conditions and interactions between these drivers. Low drought resistance and high recovery were most prominent at forest edges, but also on south-facing slopes and lower elevations. Tree functional type was the most important driver of drought resilience, with most of the damage in stands with high conifer abundance. Our results demonstrate the suitability of satellite-based quantification of drought-induced forest damage at high spatial resolution across large areas. Such information is important to predict how local site characteristics may impact forest vulnerability to future extreme events and help in the search for appropriate adaptation strategies.
极端事件，如 2018 年中欧夏季干旱，预计未来将更频繁地发生，并可能造成重大损失，包括增加树木死亡率和对森林生态系统服务的负面影响。在这里，我们根据抵抗能力、恢复能力和弹性，量化了瑞士超过 100 万个 10×10 米森林像素对 2018 年干旱的响应。我们使用 Sentinel-2 卫星数据衍生的归一化差异水指数（NDWI）作为冠层含水量的代理，并分析了其相对变化。我们计算了 2017 年干旱前和 2018 年干旱年（表示抵抗能力）之间、2018 年和 2019 年干旱后（表示恢复能力）之间以及 2017-2019 年之间（表示弹性）的 NDWI 变化。通过对这个大型自然实验的数据进行分析，我们发现瑞士森林中 4.3%的 NDWI 在 2017 年至 2018 年期间下降，这表明森林冠层对干旱影响的抵抗能力较低。虽然大约 50%的地区得到了恢复，但在 2.7%的森林地区，NDWI 从 2018 年继续下降到 2019 年，这表明持续存在负面影响或延迟的损害。我们发现，森林对干旱的反应因地形特征、森林林分特征以及气候条件和这些驱动因素之间的相互作用而有所不同。干旱抵抗力低和恢复能力高的地区最突出的是森林边缘，但也在朝南的斜坡和较低的海拔地区。树木功能类型是干旱恢复能力的最重要驱动因素，高针叶树丰度的林分中受损害最大。我们的研究结果表明，基于卫星的高空间分辨率和大面积干旱引起的森林损害量化是合适的。这种信息对于预测局部地点特征如何影响森林对未来极端事件的脆弱性以及帮助寻找适当的适应策略非常重要。
